1. Field of the Invention
This invention relates to a photopolymerizable composition comprising a polymerizable ethylenically unsaturated monomer, a photopolymerization initiator, a chlorinated polyolefin and a compound containing at least one epoxy group, and, in particular, to such a composition which has an improved stability unaccompanied by any decrease in photographic speed.
2. Description of the Prior Art
Many techniques are known to suppress thermal polymerization of a composition containing a polymerizable ethylenically unsaturated monomer. For example, various thermal polymerization inhibitors including hydroquinone, p-methoxyphenol, pyrogallol, chloranil, cuprous chloride, etc., are disclosed in Japanese Patent Publication Nos. 23761/1961, 18979/1962, 15932/1966 (corresponding to U.S. Pat. No. 3,261,686) and 43126/1973 (corresponding to U.S. Pat. No. 3,645,730).
Unfortunately, however, in photopolymerizable compositions comprising (a) an ethylenically unsaturated monomer, (b) a photopolymerization initiator represented by the general formula (I) described below and (c) a chlorinated polyolefin, it has been found that thermal polymerization is not suppressed to a satisfactory extent when one of these thermal polymerization inhibitors is used. Use of two or more thermal polymerization inhibitors and increasing the amount of the thermal polymerization inhibitors were investigated, and no desirable effect was found.
It should be noted that the use of an excess amount of the thermal polymerization inhibitor undesirably suppresses the photopolymerization of the above-described photopolymerizable composition, practically requiring a noticeable extension of exposure time to actinic radiation in the image reproduction process. This effect obviously imposes a severe restriction on the amount of thermal polymerization inhibitor which can be used. Japanese Patent Application (OPI) No. 94788/1973 discloses a method which utilizes a specialized initiator in order to improve the stability at elevated temperatures in the case of the above-described photopolymerizable composition. However, the nitroacenaphthene and nitrofluorenone compounds disclosed therein proved to exhibit insufficient activity for the initiation of photopolymerization.